This invention relates to RF-transparent electrically conductive thermal membranes or blankets for protection of antennas against thermal effects from sources of radiation such as the sun.
An antenna including a parabolic reflector can, if pointed at a source of radiation such as the sun, focus the energy from the sun onto the reflector's feed structure, possibly destroying the feed. Also, the reflector may be heated in such a manner that mechanical distortion or warping occurs, which may adversely affect proper operation.
When the antenna is mounted on a satellite as illustrated in FIG. 1, a fluence of charged particles may cause electrostatic potentials across portions of the antenna made from dielectric materials. If the potentials are sufficiently large, electrostatic discharges (ESD) may occur, resulting in damage to sensitive equipments.
A sunshield adapted for use across the aperture of a reflector antenna should significantly attenuate passage of infrared, visible and ultraviolet components of sunlight to the reflector, should have a conductive outer surface to dissipate electrical charge buildup which might result in electrostatic discharge (ESD), and should be transparent to radio-frequency signals (RF), which for this purpose includes signals in the range between the UHF band (30 to 300 MHz) and Ku band (26 to 40 GHz), inclusive.
Prior art multilayer sunshields which include plural layers of aluminized polyimide film such as Kapton film cannot be used, because they are opaque to RF at the above-mentioned frequencies. An RF-transparent multilayer blanket is described in a copending application entitled "RF-TRANSPARENT SPACECRAFT THERMAL CONTROL BARRIER", filed Dec. 5, 1990 in the names of Munro, III et al. and assigned Ser. No 07/623,144. A multilayer blanket may be disadvantageous because absorbed heat can become trapped among the several layers. The temperature of the layers rises, and they produce infrared radiation which can impinge on the reflector, thereby causing the reflector to overheat.
U.S. Pat. No. 4,479,131, issued Oct. 23, 1984 to Rogers et al., describes a thermal protective shield for a reflector using a layer of germanium semiconductor on the outer surface of a sheet of Kapton, with a partially aluminized inner surface, arranged in a grid pattern which is a compromise between RF transmittance and solar transmittance. To the extent that this arrangement allows solar transmittance, the shield and/or the reflector may heat. Such heating may not be controllable because the reflectivity of the aluminized sheet may reflect infrared from the reflector back toward the reflector, and also because both the germanium and aluminization have low emissivity.
FIG. 2 illustrates a cross-section of another RF-transparent prior art sunshield, which consists of one layer of structure. The one layer of structure includes a two-mil (0.002 inch) black Kapton film 210, reinforced with adhesively-affixed Dacron polyester mesh 212 on the side facing the reflector, and with the space-facing side painted to a thickness of about four mils with a white polyurethane paint 214 such as Chemglaze Z202, manufactured by Lord Corporation of 2000 West Grandview Boulevard, Erie, Pa. 16512. The surface of the paint is vapor coated with an electrically conductive layer 216 such as 75.+-.25.ANG. of indium-tin oxide (ITO). Such a sunshield, immediately after manufacture, has solar absorptivity .alpha., averaged over the visible spectrum, between 2.5 and 25 microns, of about 0.3, an emissivity (.epsilon.) of about 0.8, and a surface resistivity in the range about 10.sup.6 to 10.sup.8 ohms per square. It has two-way RF insertion loss of 0.24 dB.
It has been discovered that exposure of the above described single-layer sunshield to a fluence of charged particles and solar ultraviolet radiation causes a gradual degradation. The on-orbit data, together with simulation data, suggest that in the course of a 10-year mission, .alpha. increases from about 0.3 to about 0.85, and surface resistivity increases to about 10.sup.9 ohms per square. Such an increase in absorptivity may cause the single-layer sunscreen to produce infrared radiation, which may cause the antenna reflector to overheat. The increase in surface resistivity may result in ESD. New generations of satellites are intended to have mission durations much exceeding ten years, so the prior art sunscreen cannot be used. An improved sunscreen is desired.